1. Field of the Invention
The present invention relates to a shadow mask for a color CRT (Cathode Ray Tube), and particularly, to a shadow mask for a color CRT having a open type slot combining a plurality of slots in a shadow mask in order to improve the brightness of the color CRT
2. Description of the Background Art
A typical conventional color CRT is constructed as follows.
A glass envelope of the CRT is composed of a front glass panel 1 which is attached at a rear portion thereof to a glass funnel 2. Behind a screen portion of the panel 1, there is positioned a shadow mask 3 which serves as a selection filter for a fluorescent screen 4 formed on an inner surface of the panel 1. A deflection yoke 5 mounted on a neck portion of the funnel 2 causes electron beams 6 to be scanned across the screen 4 through openings formed in the shadow mask 3 to cause the screen 4 to fluoresce and emit light through the panel 1 to display an image. Typically, the openings in the shadow mask 3 are in the form of slots.
The shadow mask 3 is supported by a frame 7 which is coupled to the panel 1 by a spring 8. An inner shield 9 is also mounted to the frame, for blocking external magnetic fields. An electron gun assembly 10 is attached at the neck end of the funnel 2 for generating the electron beams 6. The envelope of the CRT is evacuated, that is, under an internal vacuum. A reinforcing band 11 is installed around a skirt of the panel 1.
The color CRT described above is operated as follows. The electron beams 6 from the electron gun mounted in the neck of the CRT impinge upon the fluorescent screen 4 formed inside the panel 1, when an anode voltage is applied. At that time, the electron beams are deflected upward/downward and left/right by a deflection yoke 5 before arriving at the fluorescent screen.
In addition, there are bipolar, tetrapolar, and hexapolar magnets 10 for correcting the paths of the electron beams 6 so that the electron beams 6 are able to impinge upon the fluorescent material precisely, whereby the deterioration of color purity in the displayed image is prevented.
Moreover, the CRT is under a high vacuum, and therefore it may be easily cracked or damaged by an external shock. In order to prevent the cracking or damage, the panel 1 is designed to have a structural strength for with standing the external atmospheric pressure.
Also, because of the reinforcing band 11 installed on a skirt of the panel 1, the stress on the CRT in the high vacuum state is dispersed.
In the color CRT adapting a tension type shadow mask, in order to improve the brightness, which is one of the most important qualities, the transmissibility of the electron beam passing through the shadow mask should be improved. In addition, in order to improve the transmissibility, the area of the openings through which the electron beam passes should be increased. In a slotted shadow mask, in order to enlarge the opening area, the width of the slot openings may be increased or the length of the slot openings may be lengthened.
If the area of the slots is enlarged, the size of the electron beams admitted therethrough are enlarged in the horizontal direction of the shadow mask when the scanned electron beams impinge upon the fluorescent material formed on the screen after passing through the slot. Therefore, allowance for landing error, which is generated when the electron beams do not impinge upon the fluorescent material precisely because the path of the electron beams is changed by external influence or by the characteristics of the color CRT, is reduced. So, when the width of the slot openings is maintained and the length of the slot openings is increased in consideration of the allowance for landing error, the electron beam transmissibility is controlled by increasing the length of the slot openings with regard to bridge shadows, which are generated by the bridges arranged between slots with a certain dimension and are seen by a user, and to Moire, that is, an interference effect of light made by compounding the spacing of the bridges dividing the slots in the vertical direction and the scanning spacing of the deflection yoke.
Also, in order to improve thermal expansion characteristics of the shadow mask, in the conventional art, the vertical pitch distance between slots is formed at twice the length, or no bridge may be formed in a column, whereby the thermal expansion characteristics of the shadow mask can be improved.
However, in the conventional color CRT including a tension type shadow mask adapting the enlarged, or so-called open type bridges in order to improve the brightness and thermal expansion characteristics, and to cope with the Moire effect phenomena, the vertical pitch, that is, the distance between the slots is very much longer than the aperture vertical pitch of other shadow masks, and therefore there may be generated problems in manufacturing the shadow mask. That is, as shown in FIG. 2, the slots are oriented perpendicularly to a rolling direction, in order to improve a creep characteristics which is one of the most important features in choosing materials of the tension type shadow mask, because a dislocation is generated inside the metal of the shadow mask and on the surface of the metal when the metal is rolled, and the dislocation surface is formed in the rolling direction.
In the dislocation surface, the mechanical characteristics, that is, a tensile strength and a stain modulus of the dislocation surface are low perpendicular to the rolling direction, but the tensile strength and the strain modulus of the dislocation surface are relatively high parallel to the rolling direction because a slip is generated between the coupled structure of the material forming the dislocation surface.
Therefore, it is desirable that the slots are formed to extend in a direction perpendicular to the direction of the rolling when the shadow mask is manufactured.
However, when the color selection electrodes are manufactured perpendicular to the direction of the rolling, there may be some problems, that is, there may result a difference in physical properties between an effective area in which the slots are formed and a non-effective area in which slots are not formed, thereby causing an enlarging of the vertical pitch distances between the slots using the open type bridge. When the shadow mask is manufactured, a tensile force is given to the metal along the rolling direction for conveying the metal, and the slots are formed extending in a direction perpendicular to the rolling direction in order to improve the creep characteristics. However, because the area of the bridges is much less than in the conventional aperture shadow mask, the bridges bordering the effective area and the non-effective area which are different in their physical properties may be broken by the concentrated force when the shadow mask is conveyed.
In addition, as the area of the bridges is small, the strength of the front surface of the shadow mask is weakened, and therefore many flanged parts and folds may be generated in the front surface of the shadow mask.
Therefore, an object of the present invention is to provide a shadow mask for a color CRT (Cathode Ray Tube) adapting a tension type shadow mask which is able to improve the thermal expansion characteristics (doming), and is able to solve the problems of deformation of the shadow mask and of slot size inequality caused by the long vertical pitch distance between slots in the shadow mask adapting the open type bridges in order to improve the brightness characteristic of the CRT.
To achieve the object of the present invention, as embodied and broadly described herein, there is provided a shadow mask for a color CRT in which a plurality of slots are formed arranged in vertical columns so that electron beams are able to pass each slot in vertical direction, a vertical pitch distance PvC between slots located around a horizontal center line of the shadow mask is greater than a distance PvE between slots located on the farthest outer part.
Also, it is desirable that the vertical pitch distance Pv between the slots is gradually shortened, so that the relation equation {fraction (1/9)}xe2x89xa6PvE/PvCxe2x89xa6⅓ is satisfied in respect of a vertical pitch distance PvC between slots around the horizontal center line of the shadow mask and a vertical pitch distance PvE between slots at horizontal edges of the shadow mask.
In addition, it is desirable that the relation ⅓xe2x89xa6PvM/PvCxe2x89xa61 is also satisfied with respect to a vertical pitch distance PvM between slots located intermediate the horizontal center line of the shadow mask and the horizontal edges of the shadow mask.
It is also desirable that a bridge width BwC between the slots located around the horizontal center line of the shadow mask is smaller than a bridge width BwE between slots located at the horizontal edges of the shadow mask.
And the present invention is also characterized in that the bridge width BwC between slots located on the center line of the shadow mask is smaller than the bridge width BwE between slots located on the horizontal edge of the shadow mask.
It is desirable that the bridge width Bw is enlarged from the horizontal center line to the edges gradually, or that the relation 1 less than BwE/BwCxe2x89xa62 is satisfied with respect to the bridge width BwC between the slots proximate the horizontal center line and the bridge width BwE proximate the edges.
Also, it is desirable that the relation 1 less than BwM/BwCxe2x89xa62 is satisfied in respect of the bridge width BwC proximate the horizontal center line and the bridge width BwM between slots located intermediate the horizontal center line and the edges.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.